> > Take, for example, the distance an object has dropped in free fall, the
> > formula is
> > h = g*t^2 / 2
> > There's nothing like air stopping the falling in this formula, it's
entirely
> > simplified, but still relevant. It just assumes that there's no
friction. I
> > was asking for the same: in a no friction-system, what's the rate of
> > slowdown for an object moving uphill?
>
> As always the key to solving a problem is to understand it.
SNIP

I don't get what you're trying to tell me, and I'm not sure that you got
what I've been trying to tell you. We have real world issues and simulation
issues that are independant of each other. Something that happens in a
simulation doesn't require the real world to behave that way and vice versa.

Now, my system can solve the problem of rolling uphill via tons of impacts
and it does look right. But how about just a formula to roll uphill,
independant of impacts? I was at a loss of the formula, not the issue
itself.

Regards,
Tim
-- 
"Tim Nikias v2.0"
Homepage: <http://www.nolights.de>
Email: tim.nikias (@) nolights.de

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Tim Nikias v2.0 wrote:
>>> Take, for example, the distance an object has dropped in free fall, the
>>> formula is
>>> h = g*t^2 / 2
>>> There's nothing like air stopping the falling in this formula, it's 
>>> entirely
>>> simplified, but still relevant. It just assumes that there's no 
>>> friction. I
>>> was asking for the same: in a no friction-system, what's the rate of
>>> slowdown for an object moving uphill?
>>
>>As always the key to solving a problem is to understand it.
> 
> SNIP
> 
> I don't get what you're trying to tell me, and I'm not sure that you got
> what I've been trying to tell you. We have real world issues and simulation
> issues that are independant of each other. Something that happens in a
> simulation doesn't require the real world to behave that way and vice versa.

Now i am stuck, could you please read the question i quoted from you 
above again and tell me if i answered your question or not?  If not 
please specify what is missing.

I have the impression that when you don't like the answer you get you 
simply say that's not what you are trying to do, that you don't want to 
simulate reality at all - fine, but then why do you ask the question in 
the first place?

> Now, my system can solve the problem of rolling uphill via tons of impacts
> and it does look right. But how about just a formula to roll uphill,
> independant of impacts? I was at a loss of the formula, not the issue
> itself.

I did not talk about impacts in my answer at all.

Christoph

-- 
POV-Ray tutorials, include files, Sim-POV,
HCR-Edit and more: http://www.tu-bs.de/~y0013390/
Last updated 11 Jan. 2004 _____./\/^>_*_<^\/\.______

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Christoph Hormann wrote:
> Now i am stuck, could you please read the question i
> quoted from you above again and tell me if i answered
> your question or not?  If not please specify what is
> missing.

If I may come with my thoughts here... I have a feeling that Tim is
creating his system in a way that resembles the way I did it.

My particle system doesn't have any such things as potential energy,
conversation of energy, or even masses. I didn't care about that. Yet I
managed to create a system where the end results looks like something
that at least resembles the real world, and where particles do in
certain ways move the way they would in the real world.

All the time while creating it my goal was not that the internal
data-representation resembled the real world, but rather that the end
result (the animations) looked (more or less) like the real world to a
certain extent. Tim Nikias probably know more about the physics of the
real world than I, but it seems he have the same attitude toward
creating a simulation as I had.

Of course it is a bit problematic in this situation to ask for help with
creating the system. Tim have to just "take it or leave it" with regards
to the proposed solutions he get, but the repliers also have to accept
that their proposed solutions might not be what Tim is looking for even
if they are physically correct...

Well, that's how I see it. I might be wrong. In any case I hope nobody
gets offended, because that's not my intention.

Rune
--
3D images and anims, include files, tutorials and more:
rune|vision:  http://runevision.com **updated Jan 29**
POV-Ray Ring: http://webring.povray.co.uk

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Rune wrote:
> 
> [...]

That's completely right, but you have to take care not to reject a 
solution just *because* it is realistic.

BTW the "it does not have to be realistic, it just has to look nice" vs. 
"if it is realistic it will look correct as well" is a very old conflict 
in CG.  But in the end it often boils down to a 
physically/mathematically correct solution - if implemented efficiently 
- also being the most useful one and the best intuitive solutions also 
being physically quite accurate.

Christoph

-- 
POV-Ray tutorials, include files, Sim-POV,
HCR-Edit and more: http://www.tu-bs.de/~y0013390/
Last updated 11 Jan. 2004 _____./\/^>_*_<^\/\.______

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> That's completely right, but you have to take care not to reject a
> solution just *because* it is realistic.

I wasn't rejecting them for being accurate, but for being near to impossible
to implement in the model I've built so far. I thought that my replies made
clear that I wasn't rejecting the physical properties, but the solutions
presented weren't fitting to my model. I tried to explain why it wouldn't
work, but I guess I've failed. Maybe we've talked alongside each other and
were interpreting too much or too little what the other was saying. Lets
just settle it with "my simulation isn't realistic, just looks like it, so
real-life formulas can't always be applied", okay? ;-)

> BTW the "it does not have to be realistic, it just has to look nice" vs.
> "if it is realistic it will look correct as well" is a very old conflict
> in CG.  But in the end it often boils down to a
> physically/mathematically correct solution - if implemented efficiently
> - also being the most useful one and the best intuitive solutions also
> being physically quite accurate.

It depends on what you're after. If you want something versatile and
intuitive, then sure, by all means, make it realistic and efficient. If you
just want one certain effect, do it quick and dirty. :-)


-- 
"Tim Nikias v2.0"
Homepage: <http://www.nolights.de>
Email: tim.nikias (@) nolights.de

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<"Tim Nikias v2.0" <tim.nikias (@) nolights.de>> wrote:

> irregularities in the reflection-direction etc). But a lying particle will
> do hundreds of passes through the loop as it will quickly bounce onto the
> surface (due to gravity) several times during the time that's passing per
> frame. That's when my System almost hangs up

So if I understand you correctly, every time a particle bounces on a surface, 
you remove some kinectic energy form it by lowering it's velocity perpedicular
to the surface.  Like v_new = -0.8*v_perpendicular + v_parallel.
Of course this leads to the situation that each bounce will get smaller (and 
will also take a shorter time) than the previous one.

One solution to this would be, that when v_perpedicular is lower than a certain
threshold just use v_new = -v_perpendicular + v_parallel, so the bounces will 
stop getting smaller. With the correct threshold the bounces should get so 
small that you just can't see them, but not smaller than that.

Lutz-Peter

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Just wanted to inform you that your idea with tracing within a timestep
works fine. I did get a few particles that traced more than 4000 iterations
of this trace-loop (the particle were rolling down a cone-tunnel, animation
will be posted shortly) but it was quick: 0.2 seconds per frame, at the
beginning and sometimes inbetween even 0.1 seconds.

Quiet a lot of bugs popped up when trying to implement this, but its finally
sorted out and I'm really satisfied with the stable results. Thanks for the
suggestion!

Regards,
Tim

-- 
"Tim Nikias v2.0"
Homepage: <http://www.nolights.de>
Email: tim.nikias (@) nolights.de

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"Tim Nikias v2.0" wrote:
> Just wanted to inform you that your idea with
> tracing within a timestep works fine.

> Quiet a lot of bugs popped up when trying to
> implement this, but its finally sorted out and
> I'm really satisfied with the stable results.
> Thanks for the suggestion!

No problem. I'll have a look at the animation. :)

Rune
--
3D images and anims, include files, tutorials and more:
rune|vision:  http://runevision.com **updated Jan 29**
POV-Ray Ring: http://webring.povray.co.uk

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This conservation of energy idea is cool.   Someone suggested it back when I
was trying to model the solar system back in 2002, but I think I got it now.


OLD SYSTEM:
    particle at   p with velocity v
    p=p+v/constant
    a=sum of forces based on new surroundings
    v=v+a/constant

    repeat for every frame of animation, giving extreme energy losses.




NEW PROPOSED SYSTEM:
    particle at p with velocity v and kinetic energy K
    p=p+v/constant
    calculate work done by moving along/against all forces in x,y, and z.
        this gives a delta to old kinetic energy K.

    okay now I'm lost

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Among other things, Greg M. Johnson wrote:

> This conservation of energy idea is cool.   Someone suggested it back when
> I was trying to model the solar system back in 2002, but I think I got it
> now.
> 
> 
> OLD SYSTEM:
>     particle at   p with velocity v
>     p=p+v/constant
>     a=sum of forces based on new surroundings
>     v=v+a/constant
> 
>     repeat for every frame of animation, giving extreme energy losses.
> 
> NEW PROPOSED SYSTEM:
>     particle at p with velocity v and kinetic energy K
>     p=p+v/constant
>     calculate work done by moving along/against all forces in x,y, and z.
>         this gives a delta to old kinetic energy K.
> 
>     okay now I'm lost

Are you looking for a way of simulating a system of "particles", subject to 
interactions between them, in a physically meaningful way? You should have 
a look at some "molecular dynamics" algorithms. Moldy 
(http://chin.icm.ac.cn/~xxia/webchin/software/moldy.html) is a nice free 
program, which features an interesting manual. Basically, you know the 
positions, velocities and forces on all the particles at a given time, and 
calculate them for a later moment, and repeat. To obtain the new positions 
(and velocities), algorithms like Verlet and "leapfrog" are popular.

-- 
light_source{9+9*x,1}camera{orthographic look_at(1-y)/4angle 30location
9/4-z*4}light_source{-9*z,1}union{box{.9-z.1+x clipped_by{plane{2+y-4*x
0}}}box{z-y-.1.1+z}box{-.1.1+x}box{.1z-.1}pigment{rgb<.8.2,1>}}//Jellby

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